Check that character range is valid

2 files changed, 17 insertions, 1 deletions

diff --git a/src/main/java/org/shedlang/compiler/sources/FileFragmentSource.java b/src/main/java/org/shedlang/compiler/sources/FileFragmentSource.java
index 2a8e6bdf..fe0c6271 100644
--- a/src/main/java/org/shedlang/compiler/sources/FileFragmentSource.java
+++ b/src/main/java/org/shedlang/compiler/sources/FileFragmentSource.java
@@ -32,11 +32,14 @@ public record FileFragmentSource(
 
     @Override
     public Source at(int characterIndexStart, int characterIndexEnd) {
+        if (characterIndexStart > characterIndexEnd) {
+            throw new IllegalArgumentException("characterIndexStart > characterIndexEnd");
+        }
+
         return new FileFragmentSource(
             filename,
             contents,
             this.characterIndexStart + characterIndexStart,
-            // TODO: check this is less than characterIndexEnd
             this.characterIndexStart + characterIndexEnd
         );
     }
diff --git a/src/test/java/org/shedlang/compiler/sources/FileFragmentSourceTests.java b/src/test/java/org/shedlang/compiler/sources/FileFragmentSourceTests.java
index ce43303c..c0d7684a 100644
--- a/src/test/java/org/shedlang/compiler/sources/FileFragmentSourceTests.java
+++ b/src/test/java/org/shedlang/compiler/sources/FileFragmentSourceTests.java
@@ -8,6 +8,7 @@ import java.util.List;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.zwobble.precisely.AssertThat.assertThat;
 import static org.zwobble.precisely.Matchers.equalTo;
 
@@ -126,4 +127,16 @@ public class FileFragmentSourceTests {
 
         assertThat(fragment, equalTo(new FileFragmentSource("<filename>", "abcdefg", 2, 5)));
     }
+
+    @Test
+    public void whenAtIsCalledWithEndBeforeStartThenErrorIsThrown() {
+        var source = FileFragmentSource.file("<filename>", "abcdefg");
+
+        var error = assertThrows(
+            IllegalArgumentException.class,
+            () -> source.at(2, 1)
+        );
+
+        assertThat(error.getMessage(), equalTo("characterIndexStart > characterIndexEnd"));
+    }
 }